Power supplies for image intensifiers utilizing the microchannel plate electron multipliers generally require expensive and complex electronic circuitry to accommodate for variations in input light levels while maintaining the output tube brightness relatively constant. A pair of oscillators, an automatic brightness control, and three high voltage multipliers are usually required along with a standard voltage clamp for good image tube operating performance. Since these circuits contain a plurality of expensive individual components it is very difficult to keep the tolerances of these individual components within the limits necessary to assure proper total circuit operation.
One method for achieving constant image tube brightness consists in detecting extremely small currents in the phosphor screen circuit. Since the variations in the phosphor screen current with changing levels of input light are very small complex circuit components are required to amplify these small current values to a useable level. The purpose of this invention therefore is to provide good image intensifier operation without the need for the complex electronic circuitry generally required for image tubes containing microchannel plate electron multipliers.